This section provides background information related to the present disclosure which is not necessarily prior art.
Flexplates, also commonly referred to as flywheels, are used in vehicular drivetrain applications to provide a mechanical coupling between an output component of an internal combustion engine and an input component of a transmission. More specifically, one side of the flexplate is secured to an engine crankshaft while the other side of the flexplate is secured to a torque converter. Consequently, the flexplate transmits engine torque to the torque converter which, in turn, transmits engine torque to the transmission.
As is widely known, the flexplate is configured to deflect axially a limited amount based on changes in engine speed, to accommodate movement and/or vibration of the crankshaft, and in response to fluid pressure variations inside the torque converter. Additionally, the flexplate serves as an engagement mechanism for the pinion gear of an electric starter motor. When the electric starter motor receives an electrical current in response to an ignition signal from the vehicle, the pinion gear engages and drives a ring gear portion of the flexplate, thereby causing the flexplate to rotatably drive the engine crankshaft. Upon the engine being successfully started, the pinion gear is disengaged while the flexplate continues to be rotatably driven by the crankshaft.
In many instances, the flexplate is a two-piece flexplate assembly comprised of a central plate and a ring gear rigidly secured to an outer rim portion of the central plate. The central plate is generally made (i.e., stamped) of a constant thickness material, although variable thickness materials can also be used. The central plate typically includes a generally flat outer portion and a slightly dished or canted central portion. The central portion includes a center aperture adapted to receive a crankshaft hub and a plurality of mounting apertures for aligning and mounting the flexplate to the crankshaft. The degree of dishing is optional and depends primarily upon the space available between the crankshaft and the torque converter. Likewise, a plurality of mounting apertures extend through the outer portion of the center plate for mounting the flexplate assembly to the torque converter.
Various methods have been employed to rigidly secure the ring gear to the central plate of conventional two-piece flexplate assemblies. For example, U.S. Publication No. US 2007/0277643 describes a two-piece flexplate assembly wherein an adhesive is used to rigidly secure the ring gear to the central plate. Additionally, U.S. Publication No. US 2007/0277644 discloses a two-piece flexplate assembly wherein the ring gear is press-fit and welded to the central plate using a gas metal arc welding (GMAW) process, commonly referred to as MIG and MAG welding. It is also known to laser weld the ring gear to the central plate.
Currently, the use of GMAW and laser welding processes are sufficient and acceptable for use in the manufacture of two-piece flexplate assemblies. However, each has certain disadvantages. Specifically, while GMAW welding is reasonably fast and inexpensive, its use may result in hardness reduction of the ring gear teeth, potential part distortion and high residual stress due to the high heat requirements. Likewise, while laser welding is a more advanced and precise process, it is inherently more expensive and complicated.
The use of these welding processes is further limited in those flexplate assemblies using a nitrided central plate. Nitrided central plates are commonly used because they permit reduce component weight as well as improve the yield and tensile strength of the central plate. However, it is difficult to consistently form a good weld between one or more nitrided parts because the weld zone can be very porous due to the release of nitrogen gas into the weld pool as the nitride layer decomposes during the welding process. Specifically, due to fast solidification of the weld pool, the nitrogen gas can become trapped and, as a result, form porous bubbles in the weld zone. To avoid this undesirable weld porosity issue, current solutions include removing the nitrided layer prior to welding and/or masking the weld surface during nitriding to establish a naked weld surface. Unfortunately, these solutions are not always commercially practical because they add extra process steps and costs to the overall production of two-piece flexplate assemblies.
Thus a need exists to develop alternative methods of welding two-piece flexplate assemblies which address and overcome these disadvantages.